Lubricant composition and mechanical element

ABSTRACT

A novel lubricant composition is disclosed. The lubricant composition comprises at least one organic compound, and, when disposed between two surfaces moving at peripheral speeds differing from each other, is capable of exhibiting a discotic columnar phase or a discotic lamellar phase in which a plurality of molecules of said organic compound is assembled.

This application claims benefit of priority under 35 U.S.C. 119 to Japanese Patent Application No. 2005-150479 filed May 24, 2005.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a lubricant composition applicable to mechanical elements used under frictional sliding, and in particular to a lubricant composition applicable to mechanical elements, excellent in a low traction performance under a condition of elastic-fluid lubrication condition.

2. Related Art

Materials used as being disposed between solid surfaces under relative motion in machines and apparatuses, aimed at reducing friction, and preventing or moderating the surface damage are referred to as lubricant, and among them, oils are most widely used as lubricant. A lubricating oil generally contains a hydrocarbon, as a base oil, obtained in a form of fraction of petroleum, and various additives added thereto depending on purposes. Although being considerably variable depending on purposes, average molecular weight of a typical base oil is known to be 350 or around, being not so largely different from that of liquid crystal used for display devices. Possibility of using the liquid crystal as lubricant has, therefore, been investigated, and in recent years, friction control making use of changes in characteristics induced by external fields has attracted a public attention, as described in Tribologist, vol. 41, No. 6, p. 506 (1996). All of these liquid crystals are, however, rod-like liquid crystals, whereas discotic liquid crystals based on the opposite concept are generally known to have high viscosity, so that low-viscosity ones operable as lubricant are not so readily available.

In another exemplary study reported on other materials, R. Eidenschink investigated alkoxytruxene exhibiting a liquid crystal phase over the temperature range from 67° C. to 293° C., only to find a friction coefficient still larger than that of the rod-like liquid crystal, as reported in Liquid Crystals, Vol. 5, pp1517 (1989). In ACS Symp. Ser., (1990) 441 (Tribol. Liq-Cryst. State), 61-82, Lauer et al. measured IR spectrum of alkoxytriphenylene exhibiting a liquid crystal phase over the range from 80° C. to to 120° C. under high shear, and suggested possibility of alignment of the discotic molecules of alkoxytriphenylene in this temperature range unlike the rod-like liquid crystal, but did not mention about magnitude of the friction coefficient. In Mol. Cryst. Liq. Cryst., 1999, Vol. 330, pp 327-334, R. Eidenschink et al. evaluated friction coefficient over the range from 60° C. to 120° C. of alkylthiobenzene exhibiting a liquid crystal phase at −10° C. or below, and reported that the compound showed a friction coefficient not so largely different from that of mineral oil after one hour or around, although some difference was observed in the friction coefficient with the elapse of time depending on presence or absence of branching in the alkyl chain.

On the other hand, there are some disclosures made on that the discotic liquid crystals show desirable lubricating performances (see Japanese Examined Patent Publication “Tokkohei” No. 2-21436, Published Japanese Translation of PCT International Publication for Patent Application “Tokuhyohei” No. 2-503326, Japanese Laid-Open Patent Publication “Tokkaihei” No. 10-279973 and Japanese Laid-Open Patent Publication “Tokkai” No. 2002-69472). “Tokkohei” No. 2-21436 and “Tokuhyohei” No. 2-503326, however, do not disclose friction coefficients under standard evaluation systems. “Tokkaihei” No. 10-279973 discloses a lubricant containing one or more species of sulfur-containing phthalocyanine or metal complex, thereby exhibiting an extremely low friction coefficient under frictional pairing, but friction coefficient of the lubricants in Examples is 0.07 at minimum, showing a value only equivalent to or larger than those of long-chain alkyl carboxylic acids, such as stearic acid, which are general oils. The present inventors have disclosed in “Tokkai” No. 2002-69472 that triazine compounds having a plurality of radial long-chain alkyl groups, wherein all of which were compounds each having a discotic center skeleton.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a lubricant composition having a low viscosity, and affording a low traction coefficient under strict extreme-pressure conditions over a wide temperature range, and to provide a low-traction mechanical element excellent in friction resistance.

In one aspect, the present invention provides a lubricant composition comprising at least one organic compound, being capable of exhibiting a discotic columnar phase or a discotic lamellar phase in which a plurality of molecules of said organic compound is assembled when disposed between two surfaces moving at peripheral speeds differing from each other.

As embodiments of the present invention, the lubricant composition having a pressure-viscosity coefficient at 40° C. of 20 GPa⁻¹ or below; the lubricant composition wherein, when disposed under shear between said two surfaces moving at peripheral speeds differing from each other, a plurality of molecules of said organic compound are aligned with their planes, exhibiting a maximum diffusion sectional area, being parallel to said two surfaces, thereby to form a molecular assembly; the lubricant composition exhibiting a minimum traction coefficient under a pressure of 10 MPa or more; the lubricant composition exhibiting a traction coefficient of 0.05 or below; the lubricant composition wherein said organic compound has at least two aromatic rings, at least one condensed ring, or a π-conjugation plane; the lubricant composition wherein said organic compound exhibits a liquid crystal phase under normal pressure; and the lubricant composition comprising two species or more of said organic compound; are provided.

In another aspect, the present invention provides a mechanical element comprising two surfaces moving at peripheral speeds differing from each other, and the lubricant disposed between said two surfaces.

According to the present invention, there is provided a lubricant composition having a low viscosity, and affording a low traction coefficient under strict extreme-pressure conditions over a wide temperature range, and there is also provided a low-traction mechanical element excellent in friction resistance.

According to the prior art related to lubricating techniques under extreme-pressure ever adopted, the lack of the strength of an oil film formed of low-viscosity, low-traction mineral oil or oil-and-fat-base synthetic lubricant has been compensated by polymer-base, sulfur-containing, or heavy-metal-base interface coating agent. According to the invention, such prior technique can be replaced with a novel technique using a material capable of exhibiting a discotic columnar phase or a discotic lamellar phase under extreme pressure and shear, to thereby allow a low viscosity to exhibit under extreme pressure. The present invention is successful not only in obtaining an extremely low traction performance superior to the conventional low traction coefficient, but also in obtaining a further tough friction resistance by using the material capable of covering the interface based on a highly-ordered alignment. In the current situation strictly demanding reduction in fuel consumption from the viewpoints of energy saving, prevention of global warming and environmental preservation, the present invention can therefore provide a novel technique contributive to solution of these problems ascribable to the Piezo-viscosity effect, which has been a big wall of scientific principle. The present invention is in no need of using any environmentally hazardous sulfur compounds, phosphorus compounds and heavy metals such as zinc, molybdenum and so forth, and is capable of providing a high-performance, environment-friendly lubricant composition by using organic compound(s) only.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing a temperature dependence of friction coefficient of LUB-1 evaluated in Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Paragraphs below will detail the present invention. It is to be understood, in this patent specification, that the term “ . . . to . . . ” is used as meaning a range inclusive of the lower and upper values disposed therebefore and thereafter.

The present invention relates to a lubricant composition used as being disposed between two surfaces moving at peripheral speeds differing from each other, and a mechanical element which includes two surfaces moving at peripheral speeds differing from each other, and the above-described lubricant disposed between two these surfaces. The peripheral speeds of the two surfaces are not specifically limited, and assuming now that the larger peripheral speed as u1 (>0), and the smaller peripheral speed as u2, or |u1|>|u2|, the average speed (u2+u1)/2 can range from not lower than zero to infinity, wherein the average speed of the two surfaces falls on 1000 m/s or below, and preferably falls in the range from 1 cm/s to 50 m/s, both ends inclusive. Also absolute value of the specific sliding Σ defined by 2×(u2−u1)/(u2+u1) can range from not lower than zero to infinity, wherein it is given as −2 for a mechanical element having a value of u2 of zero which indicates standstill, allowing use within the range of −2≦Σ<0. Most of mechanical element are, however, used within the range from −2 to −0.01, both ends inclusive.

Shear rate defined by (u1−u2)/(film thickness of lubricant composition between two surfaces) can range from not lower than zero to infinity, wherein it is generally adjusted to 10⁹/s or below, and preferably in the range from 10/s to 10⁷/s, both ends inclusive. For the case where the lubricant composition shows liquid crystallinity under normal pressure, only a small shear rate can maintain a desired highly-ordered alignment, whereas for the case of non-liquid-crystallinity, the highly-ordered alignment is attained only under shear, typically needing a shear rate of 10⁴/s or more. It is, however, difficult to unconditionally determine an appropriate range of the shear rate, because it may vary depending on structures of organic compound contained in the lubricant composition, pressure, temperature and so forth. The degree of ordered alignment of the organic compound contained in the lubricant composition, determined by the general definition for the degree of ordered alignment, is preferably adjusted to 0.3 to 0.99, both ends inclusive. The film thickness between the two surfaces is generally 10 nm to 100 μm, both ends inclusive, and preferably 50 nm to 5 μm, both ends inclusive.

As for average pressure between the two surfaces, it is described in K. E. Weale, “Ko-atsu Hanno Kagaku (High-Pressure Chemical Reaction)”, published by Baifukan, Co., Ltd. (1969), p. 1 that the organic compound generally becomes non-compressive under a pressure of approximately 100 MPa or above where the effect of “mass action” on the chemical reaction becomes less important, and that the pressure promotes the reaction overwhelming steric hindrance of the organic compound. In the mechanical element of the present invention, the above-described lubricant composition exhibits a low traction coefficient under a pressure of 10 MPa or above due to pressure rise, showing a minimum value of the traction coefficient. It is further preferable to exhibit the minimum traction coefficient under a pressure of 100 MPa or above. The lubricant composition preferably exhibits a low traction coefficient as small as 0.07 or below, and more preferably exhibits a low traction coefficient as small as 0.05 or below. It has been known that also glass and steel become affected on the interface thereof by elastic distortion, after reaching a pressure region of 10 MPa or above. It is therefore preferable that the mechanical element of the present invention performs its major movement under a pressure of 10 MPa or above, more preferably 50 MPa or above, and still more preferably 100 MPa or above. The lubricant composition is supposed to reach the mixed lubrication region as the pressure rises, and to cause fracture of the film interface. Lowering of the traction coefficient of the lubricant composition is therefore caused by pressure rise within the range from 10 MPa or above, and to a pressure where the mixed lubrication region appears.

The traction coefficient herein means a dimensionless quantity obtained by dividing the tangential force, caused when rolling is affected by slipping, by the normal force (vertical load), that is, slipping friction coefficient. As described in “Toraiborogi (Tribology)” co-authored by Yamamoto and Kaneda, published by Rikogakusha Publishing Co., Ltd., (1998) p. 129, FIG. 5.18, the traction coefficient is known to increase proportional to the slipping ratio when the slipping ratio is small, to reach a plateau thereafter, and to show a tendency of gradual decrease as being affected by friction heat when the slipping ratio further increases. The traction coefficient should, therefore, be compared under a constant temperature, and in a region of a relatively large slipping ratio where a maximum traction coefficient can be obtained.

In general fluid lubrication action, a narrower gap results in generation of a larger pressure, so that concentrated contact such as point contact and line contact caused by roller bearing, gear and cam produces a pressure of several hundred MPa to Gpa. Not only elastic deformation of the interface per se, but also viscosity of the lubricating liquid exponentially increase with pressure. The relation between the pressure and the viscosity of lubricating liquid is given by the Barus equation below: η=η₀ exp(αP)  (1) wherein logarithmic conversion of both sides gives: logη=logη₀+loge×αP  (2) and logarithmic η and pressure P are in a linear relation with gradient α. The index α of pressure dependence of viscosity is called as pressure-viscosity coefficient.

The lubricant composition of the present invention preferably has a pressure-viscosity coefficient at 40° C. of 20 GPa⁻¹ or below, and preferably exhibits a low traction coefficient with increase in pressure under a pressure of 10 MPa or above. The pressure-viscosity coefficient herein can be calculated according to the method described in Tribologist, Vol. 38, No. 10, pp 927 (1993). The lubricant composition of the present invention preferably has a pressure-viscosity coefficient at 40° C. of 13 GPa⁻¹.

For any compound existing in a solid form at 40° C., the pressure-viscosity coefficients are measured at two or more temperatures where the compound liquefies, and the obtained values are extrapolated towards the low temperature side to thereby define the value at 40° C.

The lubricant composition of the present invention contains at least one species of organic compound, and can form a discotic columnar phase or a discotic lamellar phase in which a plurality of molecules of the organic compound(s) is assembled when disposed between two surfaces moving at peripheral speeds differing from each other. It is to be understood herein that the term “organic compound” in the present invention is used for any metal complexes having an organic compound(s) as ligand thereof. In general, it is not easy to structurally specify molecules which can form a discotic self-assembly. Various factors such as a steric structural factor and an inter-molecular interaction may contribute to forming the discotic self-assembly. A plurality of molecules may interact with each other based on specific or position-selective, inter-molecular interaction (a kind of molecular identification action) such as hydrogen bond or metal-metal interaction, thereby to form a discotic self-assembly, as if one disc is formed as a whole. And rod-like molecules, having a specific balance in sizes and geometries of the non-polar, long-chain portion and the polar core portion thereof, may form a d a discotic self-assembly. These phenomena are specifically reported as columnar phase formation by an oligomer of an oligoethyleneoxy ester-base compound of trialkoxy gallic acid as described in J. Chem. Soc., Perkin Trans., 2, 1994, 31; columnar phase formation by a tetraalkoxyphenanthridinone dimer as described in Angew. Chem., Int. Ed. Engl., 1995, 34, No. 15, 1637; columnar phase formation by a glucitol amide-base compound of trialkoxygallic acid as described in Liquid Crystals, 1997, Vol. 22, No. 4, 427; formation of super-liquid crystal phase of barbitul acid as described in Crystals, 1997, Vol. 22, No. 5, 579; columnar phase formation by rhodium- or iridium-containing dicarbonyl β-diketonate complex described in Chem. Mater., 1998, 10, 438; and columnar phase formation by a intra-molecular hydrogen bond complex of triaryl melamine and aromatic carboxylic acid as described in Liquid Crystals, 1998, Vol. 24, No. 3, 407. Any of these materials causative of these phenomena, and any materials analogous to these materials and supposed to be causative of similar phenomena are applicable to the lubricant composition of the present invention. It is, however, to be noted that J. Am. Chem. Soc., 1997, Vol. 119, No. 18, 4097 describes that trialkoxy stilbazole trimer of trimellitic acid and phthalimide trimer of melamine cannot readily be formed, suggesting that formation of the columnar phases of these compounds are based on some delicate structural factors.

It is preferred that the organic compound, which can be employed in the lubricant composition of the present invention, has at least two aromatic rings (more preferably benzene rings) , at least one condensed ring (more preferably an aromatic condensed ring, such as naphthalene ring or phenanthrene ring), or π-conjugation plane. The organic compound is preferably selected from materials showing a liquid crystal phase under normal pressure. Some organic compounds, even being incapable of forming a discotic columnar phase nor a discotic lamellar phase all by themselves, may form the phase if mixed with any other organic compounds, so that the embodiments of the present invention also include the lubricant composition comprising a plurality of species of organic compounds, being capable of exhibiting a discotic columnar phase or a discotic lamellar phase in which molecules of such plurality of species of organic compounds are assembled.

It is preferred that, when the lubricant composition of the present invention disposed under shear between the two surfaces moving at peripheral speeds differing from each other, molecules of the organic compound(s) are aligned with their planes, exhibiting a maximum diffusion sectional area, being parallel to the two surfaces, thereby to form a molecular assembly film. For the case of rod-like molecules, they are aligned with their inertial axes or optical axes being parallel to the two surfaces, thereby to form a molecular assembly film. For the case of planar or discotic molecules, they are aligned with their largest molecular planes being parallel to the two surfaces, thereby to form a molecular assembly film.

The organic compound may be selected from discotic organic compounds having a plurality of radial side chains. This sort of compound can ensure a large free volume of the side chains even in the self-assembly structure, and is consequently understood as a large-free-volume compound, or a low-viscosity compound under high pressure, and is expected to show a low friction coefficient under conditions of elastic fluid lubrication. In fact, the present inventors have confirmed that discotic liquid crystals, having viscosity values considerably larger than those of general lubricant base oils under normal temperature and normal pressure, respectively showed extremely low friction coefficients under conditions of elastic fluid lubrication, and have measured the pressure-viscosity coefficient of a triazine-base discotic compound according to a known method described in Liquid Crystals, 1997, Vol. 22, No. 4, 427, and have confirmed that the compound showed a value as small as being comparative to those of oil-and-fat compounds. It is therefore supposed that, by using such a discotic compound, a film of a discotic structurally-assembly having side chains radiating therefrom is formed between the two surfaces, and that an appropriate level of low friction coefficient, attributed to the large free volume of its chemical structure, can be obtained under conditions of elastic fluid lubrication, or high pressure and high shear.

Specific examples of the organic compounds, which can be employed in the present invention, include, however not to be limited to, compounds listed below.

In view of ensuring practical performances adaptive to various applications, the lubricant composition of the present invention may optionally be added with various additives generally used for lubricant including bearing oil, gear oil and power transmission oil, that is, anti-wearing agent, extreme pressure agent, antioxidant, viscosity index improver, detergent-dispersant, metal deactivator, anticorrosive, anti-rust agent, defoaming agent and so forth, within ranges without impairing the purpose of the present invention. The lubricant composition of the present invention may contain a medium for the organic compound. The medium can be selected from any materials so far as they do not inhibit spontaneous formation of the self-organizing material. For example, one species, or two or more species may be selected from general mineral oils and synthetic oils which have conventionally been used as lubricant base oils. It is to be noted that, in any case of containing other constituents, such one species or two or more species of organic compounds preferably account for 50 mol % or more of the total composition, and more preferably 80 mol % or more.

Structure of the mechanical element of the present invention is not specifically limited so far as it has two surfaces moving at peripheral speeds differing from each other, and the above-described lubricant disposed between two these surfaces. It may be any of mechanical elements incorporated into conventionally-known frictional sliding portions in need of lubricant, grease and so forth. The two surfaces moving at different peripheral speeds may be curved surfaces or flat surfaces, or may be such as having irregularity over the entire surface or in a part thereof. Examples of which include frictional sliding portions of sliding bearing and rolling bearing. The mechanical element of the present invention may further include gear, cam, screw or traction drive as a transmission element. The mechanical component may still further include a contact seal such as oil seal, mechanical seal, piston ring, or the like, as a sealing element sealing the lubricant composition.

Materials of the two moving surfaces can be exemplified by carbon steel for mechanical structures; alloy steels for mechanical structures such as nickel-chromium steel, nickel-chromium-molybdenum steel, chromium steel, chromium-molybdenum steel, and aluminum-chromium-molybdenum steel; stainless steel; multi-aging steel; ceramics such as silicon carbide, silicon nitride, alumina and zirconia; cast iron; copper, copper-lead, aluminum alloy and castings thereof; white metal; various plastics including high-density polyethylene (HDPE), polytetrafluoroethylene resin (PFPE) polyacetal (POM), polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyamide imide (PAI) and polyimide (PI); organic-inorganic composite materials composed of plastic compounded with fiber such as those composed of glass, carbon, aramid and so forth; and cermet as a composite material of ceramic and metal.

EXAMPLES

Paragraphs below will more specifically explain the present invention referring to Examples and Comparative Examples, wherein the present invention is by no means limited to these Examples. Evaluation of the lubricant composition in Examples and Comparative Examples was carried out according to the methods below.

-   1. Methods of Evaluation and Measurement Using Reciprocating (SRV)     Friction Wear Test

Friction coefficient and wear resistance were evaluated using a reciprocating (SRV) friction wear tester, wherein the friction wear test was carried out according to the test conditions shown below.

Test Conditions

-   -   Test piece (friction member): SUJ-2     -   Surface roughness: 0.45 to 0.65 μm     -   Plate: 24 mm in diameter×7 mm     -   Cylinder: 15 mm in diameter×22 mm     -   Temperature: 0 to 150° C.     -   Load: 100 N (149 MPa)     -   Amplitude: 1.5 mm     -   Frequency: 50 Hz     -   Preliminary slide time: 2 minutes

-   2. Lubricant Composition using Compound Capable of Forming Discotic     Assembly Structure

Example 1

A lubricant LUB-1 having a composition below was evaluated by the reciprocating (SRV) friction wear test. Results are shown in FIG. 1. Composition of LUB-1 and phase transfer temperatures are shown below.

Composition of LUB-1

Isomolar mixture of a and b

Liquid Crystal Phase Transfer Temperatures of LUB-1

LUB-1 is a composition exhibiting liquid crystallinity under normal pressure and normal temperature, and showed a property of being readily aligned under shear. Molecules of discotic compounds are horizontally aligned with respect to a polar surface, unless the surface is specifically treated for allowing thereon vertical alignment. Under the conditions of this experiment, LUB-1 is therefore supposed to form a molecular assembly film in which molecules are aligned with their molecular plane, having a maximum diffusion sectional area, being parallel to the shear plane. Changes in the phase transfer temperature could otherwise be observed also under pressure, but alignment property under shear preferably remains unchanged or evolves into a higher order degree from the viewpoint of energy, so that LUB-1 is supposed to show a tendency of being more readily aligned. Exhibition of a low traction coefficient at a temperature region higher than the isotropic phase transfer temperature under normal pressure, as shown in FIG. 1, suggests that LUB-1 is in a highly-aligned state even at such temperature.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a mechanical element exhibiting a low traction coefficient under frictional sliding with elastic fluid lubrication, such as piston, bearing, cam, gear, transmission, and bone joint in biological systems, and to provide a lubricant composition applicable to the mechanical element. 

1. A lubricant composition comprising at least one organic compound, being capable of exhibiting a discotic columnar phase or a discotic lamellar phase in which a plurality of molecules of said organic compound is assembled when disposed between two surfaces moving at peripheral speeds differing from each other.
 2. The lubricant composition of claim 1, having a pressure-viscosity coefficient at 40° C. of 20 GPa⁻¹ or below.
 3. The lubricant composition of claim 1, wherein, when disposed under shear between said two surfaces moving at peripheral speeds differing from each other, a plurality of molecules of said organic compound are aligned with their molecular planes, having a maximum diffusion sectional area, being parallel to said two surfaces, thereby to form a molecular assembly.
 4. The lubricant composition of claim 1, exhibiting a minimum traction coefficient under a pressure of 10 MPa or more.
 5. The lubricant composition of claim 1, exhibiting a traction coefficient of 0.05 or below.
 6. The lubricant composition of claim 1, wherein said organic compound has at least two aromatic rings, at least one condensed ring, or a π-conjugation plane.
 7. The lubricant composition of claim 1, wherein said organic compound exhibits a liquid crystal phase under normal pressure.
 8. The lubricant composition of claim 1, comprising two species or more of said organic compound.
 9. A mechanical element comprising two surfaces moving at peripheral speeds differing from each other, and a lubricant as set forth in claim 1 disposed between said two surfaces. 